Modern high-throughput radiocommunications satellites offer high transmission capacities, of the order of hundreds of Gbps (100 to 600 Gbps), by virtue of the use of thin antenna beams, combined with spatial frequency resource reuse in a fixed wide band and with an effective strategy for the adaptive modulation and coding of the transmission channels.
Optimizing transmission capacity is a major challenge, and frequency reuse techniques are a key element thereof.
Frequency reuse techniques are known that are commonly called two-colour, four-colour or FFR for fractional frequency reuse colour schemes.
Among the conventional frequency reuse schemes, the scheme that corresponds to an allocation of four separate sub-bands over all of the coverage cells or spots of the multibeam antenna in accordance with a four-colour pattern is well known. A four-colour frequency reuse scheme, denoted using the acronym 4-FR for “4 colours frequency”, divides the entire frequency band allocated to the system into four separate frequency sub-bands. A colour corresponds to a sub-band and an associated polarization (out of the two possible polarizations: Right Hand Circular Polarization RHCP and Left Hand Circular Polarisation LHCP).
A 4-colour colour scheme 4-FR allows adjacent beams of the transmit or receive satellite antenna to be transmitted on the various sub-bands on an outward channel from the satellite to the ground, or to be received on the various sub-bands on a return channel from the ground to the satellite. A four-colour frequency reuse scheme 4-FR makes it possible to have a constant minimum inter-beam distance between beams of the same colour, and therefore to achieve a reasonable compromise between the reuse factor of the frequency band and the isolation between the beams so as to limit interference.
Also known is a two-colour frequency reuse scheme, which leads to very high levels of interfering elements.
FIGS. 1, 2 and 3 respectively show the frequency plans for a two-colour, a four-colour and a fractional frequency reuse FFR colour scheme.
As illustrated in FIGS. 4a and 4b, a 4-colour fractional frequency reuse (FFR) scheme is also known (there is a mixture of 4 colours at the spot border and 2 colours at the spot centre (which is simply the joining of the 2 colours of like polarization of one polarization of a 4-colour scheme)), wherein a sub-band common to all of the cells is allocated to an inner zone of each cell Cell.
FIG. 4a corresponds to hexagonal cells, and FIG. 4b corresponds to square cells.
A first polarization state and a second polarization state are respectively allocated to the cells of a first grid G1 and to the cells of a second grid G2.
The cells of the first grid G1 have a colour A allocated to a central inner zone of each cell, and two other colours B and C respectively allocated to the other non-central part of each cell, such that two contiguous cells of the first grid G1 do not have the same colour B or C in their respective outer part.
Similarly, the cells of the second grid G2 have a colour D allocated to a central inner zone of each cell, and two other colours E and F respectively allocated to the other non-central part of each cell, such that two contiguous cells of the second grid G2 do not have the same colour E or F in their respective outer part.
The colours A, B, C and D, E, F generally correspond to the same division into frequency sub-bands; only the polarizations are different.
In FIG. 4a, with hexagonal cells, the central part, indexed a, of a cell is taken as reference, whose significant interfering elements are the central parts, indexed b, of two cells, whose less significant interfering elements are the central parts, indexed c, of two cells, and whose even less significant interfering elements are the central parts, indexed d, of four cells.
In FIG. 4b, with square cells, the central part, indexed a, of a cell is taken as reference, whose significant interfering elements are the central parts, indexed b, of four cells, and whose less significant interfering elements are the central parts, indexed c, of four cells.